The present invention relates to human foreskin cells, which are capable of maintaining stem cells in an undifferentiated state in culture. Embryonic stem (ES) cells are derived from the inner cell mass (ICM) of the mammalian blastocyst (Evans & Kaufman 1981; Martin 1981). These cells are pluripotent thus capable of developing into any organ or tissue type and even a complete embryo. When cultured in suspension ES cells aggregate and differentiate into simple embryoid bodies (EBs), however once cultured under suitable conditions (as further described hereinbelow), true ES cells are capable of indefinite proliferation in vitro in an undifferentiated state; maintaining a normal karyotype through prolonged culture; and maintaining the potential to differentiate to derivatives of all three embryonic germ layers (i.e., mesoderm, ectoderm and endoderm).
Mouse embryonic stem cells provide a powerful tool for introducing specific genetic changes into the mouse germ line.
Mouse ES cells combined into chimeras with normal preimplantation embryos and returned to the uterus participate in normal embryonal development [Richards (1994) Cytogenet. Cell Genet. 65: 169-171]. The ability of mouse ES cells to contribute to functional germ cell in chimeras provides a method for introducing site specific mutations into mouse lines. For example, with appropriate transfection and selection strategies, homologous recombination can be used to derive ES cell lines with planned alterations of specific genes. The genetically altered cells can be used to form chimeras with normal embryos and chimeric animals are recovered. Once ES cells contribute to the germ line in the chimeric animal, it is feasible to establish a mouse line for the planned mutation in the next generation. Thus, mouse ES cells provide refined mutagenesis screen that greatly accelerate functional mouse genomics and generate mammalian models for developmental processes and disease (Mills and Bradley, 2001).
Although mouse ES cells facilitate the understanding of developmental processes and genetic diseases, significant differences in primates and mouse development limit the use of mouse ES cells as a model of human development. Mouse and primate embryos differ significantly in temporal expression of embryonic genes, such as in the formation of the egg cylinder versus the embryonic disc [Kaufman, The Atlas of Mouse Development; London; Academic Press (1992)]; in the proposed derivation of some early lineages [O'Rahilly and Muller; Developmental Stages in Human Embryos, Washington; Carnegie Institution of Washington (1987)]; in the structure and function of the extraembryonic membranes and placenta [Mossman, Vertebrate Fetal Membranes; New Bruswick; Rutgers (1987)]; in growth factor requirement for development (e.g., the hematopoietic system) [Lapidot Lab. An. Sci. 43:147-149 (1994)]; and in adult structure and function (e.g., central nervous system).
Thus, to better reflect developmental differences, ES cells have also been generated from primates (Thomson et al., 1995, 1996, 1998).
Human ES cells offer insight into developmental events, which cannot be studied directly in the intact human embryo. For example, in the early post-implantation period, knowledge of normal human development is largely restricted to the description of a limited number of sectioned embryos and to analogies drawn from experimental embryology of other species.
Furthermore, screens based on the in vitro differentiation of human ES cells to specific lineages can identify gene targets, which can be used for the design and configuration of tissue regeneration therapies and teratogenic or toxic compounds.
For example, Parkinson's disease and juvenile-onset diabetes mellitus, result from the death or dysfunction of one or several cell types. Replacement of non-functional cells using ES cells technology can offer a lifelong treatment.
In order to maintain human ES cells in an undifferentiated state ES culture must be supplemented with factors which maintain cell proliferation, inhibit ES cell differentiation and preserve pluripotency. Current methods for culturing ES cells include the use of mouse feeder cells or conditioned medium. Other methods aim to provide an animal-free environment for the growth of human ES cells.
Animal Based Cultures
Animal based cultures include mouse feeder layers supplemented with serum or serum replacement and mouse originated matrices supplemented with conditioned medium.
Mouse Feeder Layers
The most common method for culturing ES cells is based on mouse embryonic fibroblasts (MEF) as a feeder cell layer supplemented with tissue culture medium containing serum or leukemia inhibitor factor (LIF) which supports the proliferation and the pluripotency of the ES cells (Thomson et al, 1998; Reubinoff et al 2000). MEF cells are derived from day 12-13 mouse embryos in medium supplemented with fetal bovine serum. Under these conditions ES cells can be maintained for many passages in culture while preserving their phenotypical and functional characteristics. However, unlike mouse ES cells, the presence of exogenously added LIF does not prevent differentiation of the human ES cells. Furthermore, the use of feeder cells substantially increases the cost of production, and makes scale-up of human ES cell culture impractical. Additionally, the feeder cells are metabolically inactivated to keep them from outgrowing the stem cells, hence it is necessary to have fresh feeder cells for each splitting of the human ES culture. Procedures are not yet developed for completely separating feeder cell components away from embryonic cells prepared in bulk culture. Thus, the presence of xenogeneic components from the feeder cells complicates their potential use in human therapy.
ES cells can also be cultured on MEF under serum-free conditions using serum replacement supplemented with basic fibroblast growth factor (bFGF) (Amit et al., 2000). Under these conditions the cloning efficiency of ES cells is 4 times higher than under fetal bovine serum. In addition, following 6 months of culturing under serum replacement the ES cells still maintain their pluripotency as indicated by their ability to form teratomas which contain all three embryonic germ layers. Although this system uses a better-defined culture conditions, the presence of mouse cells in the culture exposes the human culture to pathogens which restricts their use in cell-based therapy.
Conditioned Medium
ES cells can also be cultured in a feeder-free environment. Stem cells are grown on a solid surface such as an extracellular matrix (e.g., Matrigel™ or laminin) in the presence of culture medium. The culture medium used for growing the stem cells contains factors that effectively inhibit differentiation and promote their growth such as MEF-conditioned medium and bFGF. However, this culturing method is limited by the high costs of both the matrix and the production of MEF conditioned medium. In addition, both the matrix and the conditioned medium consist of mouse material, which is basically inconsistent in terms of composition.
However, the major disadvantage of all the above mentioned animal-based xenosupport systems (i.e., MEF with serum and serum replacement, extracellular matrices and conditioned medium) is that they present the risk of animal pathogen cross-transfer to the human ES cells, thus compromising future clinical application.
Animal-free Cultures
Animal free cultures provide a pathogen-free environment for the growth of ES cells. These cultures rely on human feeder layers supplemented with human serum or serum replacement suitable for the growth of human stem cells.
Human Feeder Layer
Human ES cells can be grown and maintained using human embryonic fibroblasts or adult fallopian epithelial cells. When grown on human feeder cells the human ES cells exhibit normal karyotypes, present alkaline phosphatase activity, express Oct-4 and other embryonic cell surface markers including SSEA-3, SSEA-4, TRA-1-60, and GCTM-2, form teratomas in vivo, and retain all key morphological characteristics (Richards et al 2002). However, the major disadvantage of using human embryonic fibroblasts or adult fallopian tube epithelial cells as feeder cells is that both of these cell lines have a limited passage ability of only 8-10 times, thereby limiting the ability of a prolonged ES growth period. For a prolonged culturing period, the ES cells must be grown on human feeder cells originated from several subjects which results in an increased variability in culture medium.
There is thus a widely recognized need for, and it would be highly advantageous to have, an animal-free culturing system, capable of supporting stem cell proliferation in culture for extended periods of time, while maintaining their undifferentiated state, devoid of the above limitations.